Irbesartan is chemically, 2-butyl-3-[[2′-(1H-tetrazol-5-yl)[1,1′-biphenyl]-4-yl]-1,3-diazaspiro[4,4]non-1-en-4-one and is represented by the Formula I

Irbesartan and its use in treating hypertension were first disclosed by Bernhart et al in U.S. Pat. No. 5,270,317 and assigned to Sanofi. The process for the preparation of irbesartan described in '317 patent involves the formation of tetrazole ring by a reaction of the cyano intermediate (Formula II) with tributyltin azide.

The '317 process yields irbesartan of low purity and requires purification by column chromatography.
The application WO 2005/113518 describes a process for irbesartan where the cyano intermediate (Formula II) is reacted with tributyltin chloride and sodium azide. The application WO 2005/051943 describes a process using tributyltin chloride, sodium azide and tetrabutyl ammonium bromide (TBAB) to prepare irbesartan. The use of tributyltin reagents results in a product with high content of tin and requires extensive purification steps.
The patent EP 1918288A1 describes a process for the preparation of irbesartan which involves the reaction of the cyano intermediate with sodium azide and zinc chloride. Irbesartan obtained by using zinc chloride was found to contain significant amounts of 2-n-butyl-3-[(2′-carboxamidobiphenyl-4-yl)methyl]-1,3-diazaspiro-[4,4]-non-1-en-4-one (Formula-III) as an impurity. The removal of this impurity requires extensive purification resulting in considerable yield loss.

Synthesis of tetrazole moiety from a cyano compound using sodium azide and triethylammonium chloride in DMF is reported in the literature (Synthesis, 1987, 1133-1134). The U.S. Pat. No. 5,629,331 also describes a process for irbesartan where the cyano intermediate is reacted with sodium azide and methyl ammonium chloride in 1-methyl pyrrolidin-2-one as a solvent. Irbesartan obtained by this method does not have satisfactory purity and also contains the 2-n-butyl-3-[(2′-carboxamidobiphenyl-4-yl)methyl]-1,3-diazaspiro-[4,4]-non-1-en-4-one (Formula-III) as an impurity. Extensive purification is required for the removal of this impurity. The solvent used is also expensive. PCT application WO 2007/013101 describes the use of triethylamine and acetic acid in place of triethylammonium chloride. However, it is reported in NO 2008/107799 that the Irbesartan obtained by the process described in the '101 application does not have satisfactory purity and the process produces poor product yield. Moreover the processes using triethylammonium chloride require long times of reaction and yet result in incomplete conversion.
Thus the processes described in the prior art are not satisfactory on an industrial scale. There is a need for an improved process for the preparation of irbesartan that is economical and results in high purity of the product.